python读取文件时，删除重复行并计数

from collections import Counter

with open('a.txt', 'r+') as f:
    a = f.readlines()
    for i in range(len(a)):
        a[i]  = a[i].strip().lower()

    b = []
    a = dict(Counter(a))
    for i in a.keys():
        b.append('%s %d\n' % (i, a[i]))

f = open('a.txt', 'w')
f.writelines(b)